Amplification circuit which includes an input-current compensation device

ABSTRACT

An amplification circuit comprising a device for compensating for its input current. The amplification circuit includes an amplifier and a capacitor located on the input side of the amplifier. The input-current compensation device consists of a current generator, generating a current Ig, a switch and a device for measuring the average current charging or discharging the capacitor during the time that the switch is open. The current Ig is such that: Average current=Ia-Ig, where Ia is the input current of the amplifier. The value of the average current is close to zero. The value of the current Ig which the current generator must output, so as to make the average current close to zero, is calculated using the device for measuring the average current. The invention particularly applies to a circuit for restoring the DC component of a video signal.

BACKGROUND OF THE INVENTION

The invention relates to an amplification circuit and more particularlyto an amplification circuit which includes a device for compensating forits input current.

The invention will be more particularly described within the frameworkof the restoration of the DC component of a video signal.

However, as will emerge later, the invention relates to otherapplications such as, for example the stabilization of a voltage.

As is known to those skilled in the art, the DC component of the videosignal delivered by an image sensor is not transmitted by the processingcircuits located on the output side of the image sensor. A circuit forrestoring the DC component of a video signal is therefore necessary.

The circuit in FIG. 1 represents a diagram showing the principle of acircuit for restoring the DC component of a video signal according tothe prior art.

The circuit in FIG. 1 includes a capacitor C, commonly called a clampingcapacitor, a switch K and an amplifier A1.

The switch K has an on-state resistance R_(ON), the value of which istypically less than or equal to 100 and an off-state resistance ofvirtually infinite value.

The video signal VE is applied to a first plate of the capacitor C, thesecond plate of which is connected to the input of the amplifier A1. Afirst terminal of the switch K is connected to the common pointconnecting the second plate of the capacitor C and the input of theamplifier A1, and a second terminal of the switch K is connected to theearth of the circuit. As is known to those skilled in the art, theoutput voltage VS1 of the amplifier A1 is preferably equal to zerovolts.

When the switch K is closed, the second plate of the capacitor isearthed, to the earth of the circuit, with the time constant τ=C×R_(ON)As is known to those skilled in the art, the switch K is closed by theaction of a control pulse generated from a sync signal.

The switch is closed for a time T lying within the line blankinginterval, after the trailing edge of the sync signal. The time τ isadjusted so as to be very much shorter than the time T, which is about 3to 4 μs. It follows that the input voltage of the amplifier A1 isclamped to earth at each line, whatever the value of the potentialpresent on the input of the amplifier A1 at the moment when the switch Kcloses.

Between two line sync pulses, the capacitor C behaves as a battery anddelivers its input current Ia to the amplifier A1. The variation involtage ΔV_(c) which then appears across the terminals of the capacitorC is such that:

ΔV_(c) =ΔQ/C, with ΔQ=Ia×T_(L) where T_(L) represents the timeseparating two line sync pulses.

It follows therefore that:

    ΔV.sub.c =T.sub.L ×Ia/C.

In order to minimize the value of ΔV_(c), the amplifier A1 is chosen soas to have a high input impedance. It follows that the current Ia issmall and that the voltage variation ΔV_(c) can then be neglected.

Thus, according to the known prior art, the input stage of the amplifierA1 is produced either using one or two junction field-effect transistors(JFET) or using one or two field-effect transistors of the MOSFET type,the JFET and MOSFET transistors having virtually infinite inputimpedances.

The use of JFET or MOSFET transistors has, however, many drawbacks.

This is because JFET transistors, apart from the fact that they areexpensive, have a high consumption, typically about 30 mA, and, fortheir supply, they require a format of voltages of relatively high value(typically between +12 volts and -12 volts).

With regard to MOSFET transistors, the noise voltage density which theygenerate is very high, about 20 to 25 nV per Hz, thereby preventing anyuse of these transistors for professional applications.

The invention does not have such drawbacks.

BRIEF SUMMARY OF THE INVENTION

This is because the present invention relates to an amplificationcircuit which includes an amplifier, having an input and an output, anda capacitor, having a first plate and a second plate, the second plateof the capacitor being connected to the input of the amplifier and thefirst plate of the capacitor being connected to the input voltage of theamplification circuit. The amplification circuit includes means formaking the average current ie flowing through the capacitor more or lessequal to zero, whatever the value of the input current Ia of theamplifier.

The means for making the average current ie flowing through thecapacitor to be approximately equal to zero consist of at least onecurrent generator Ig and of a device for measuring the average currentie so that ie=Ia-Ig.

One advantage of the invention is that it avoids the use of JFET orMOSFET transistors.

Other characteristics and advantages of the invention will appear onreading a preferred embodiment given with reference to the figuresappended hereto, among which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a simplified diagram of a circuit for restoring the DCcomponent of a video signal according to the prior art;

FIG. 2 represents a simplified diagram of an amplification circuitaccording to the invention, applied to a circuit for restoring the DCcomponent of a video signal;

FIG. 3 represents a detailed description of the simplified diagram shownin FIG. 2;

FIG. 4 represents a detailed description of an amplification circuitaccording to the invention, applied to a voltage stabilization circuit;and

FIG. 5 represents an improvement of a part common to the circuits shownin FIGS. 3 and 4.

In all the figures, identical references denote the same components.

DETAILED DESCRIPTION

FIG. 2 shows the simplified diagram of an amplification circuitaccording to the invention, applied to a circuit for restoring the DCcomponent of a video signal.

The amplification circuit according to the invention includes a clampingcapacitor C, a switch K, a current generator 1, an amplifier A2 havingan input current Ia and a device 2 for measuring the average value ofthe current ie flowing through the capacitor C. According to thepreferred embodiment of the invention, the output voltage VS2 of theamplifier A2 is equal to zero volts.

The video signal VE is applied to a first plate of the capacitor C, thesecond plate of which is connected to the input of the amplifier A2, toa first terminal of the switch K and to a first terminal of the currentgenerator 1. The second terminal of the switch K is connected to a firstterminal of the device 2, the second terminal of which is connected tothe earth of the circuit. The second terminal of the current generatoris connected to the earth of the circuit.

The current Ig delivered by the current generator 1 is such thatie=Ia-Ig, where ie is the current charging or discharging the capacitorC during the time that the switch K is open.

According to the preferred embodiment of the invention, the value of thecurrent ie is chosen so as to be close to zero. The value of the currentIg must therefore be a value very close to that of the current Ia.

According to the invention, when the switch K is open, the capacitor Cis charged by the very small current ie. It follows that the currentwhich flows through the closed switch K is also a small current.Advantageously, the variations in the input voltage of the amplifier A2are therefore negligible.

The value of the current Ig which the current generator 1 must output iscalculated from the measurement of the average value of the current ie.The calculation of the current ie is shown in detail in FIG. 3.

FIG. 3 represents a detailed description, according to the preferredembodiment of the invention, of the simplified diagram shown in FIG. 2.

The device for measuring the average value of the current ie consists ofthe differential amplifier 3, the positive and negative inputs of whichare respectively denoted by e(+) and e(-), the resistor Rg, thecapacitor Cg, the transistors T3 and T4 and the resistor R5.

The transistors T3 and T4 are transistors respectively of the npn andpnp type which have their bases connected together and to the output ofthe differential amplifier 3. The emitter of the transistor T3 isconnected to the emitter of the transistor T4 and to a first terminal ofthe resistor R5, the second terminal of which is connected to the earthof the circuit. The first terminal of the resistor RS is also connected,on the one hand, to the first terminal of a resistor Rg, the secondterminal of which is connected to the input e(-) of the differentialamplifier 3 and, on the other hand, to the first terminal of a capacitorCg, the second terminal of which is connected to the input e(-).

Preferably, the differential amplifier 3 has a zero offset voltage. Itfollows that the average potential present on the input e(-) is zero.

The differential amplifier 3 has its input e(-) connected to the secondterminal of the switch K and its input e(+) connected to the earth ofthe circuit.

FIG. 3 corresponds to the special case in which the current Ia is acurrent input into the amplifier A2.

In general, the current Ia to be compensated for may be either a currentinput into the amplifier A2 or a current output by the amplifier A2.

In order to be able to compensate for input or output currents Ia, thecurrent generator consists of two current sources of the current-mirrortype.

The first current source consists of the transistor T1, the resistors R1and R2 and the diode D1.

The second current source consists of the transistor T2, the resistorsR3 and R4 and the diode D2.

The collectors of the transistors T1 and T2 are connected together andare connected to the second plate of the capacitor C and to the input ofthe amplifier A2.

The emitter of the transistor T1 is connected via the resistor R1 to thepositive voltage +VA of the power supply for the amplifier A2.

Likewise, the emitter of the transistor T2 is connected via the resistorR3 to the negative voltage -VA of the power supply for the amplifier A2.

The collector of the transistor T3 is connected to the base of thetransistor T1, which base is connected to the cathode of the diode D1whose anode is connected to the first terminal of the resistor R2, thesecond terminal of which is connected to the voltage +VA.

The collector of the transistor T4 is connected to the base of thetransistor T2, which base is connected to the anode of the diode D2whose cathode is connected to a first terminal of the resistor R4, thesecond terminal of which is connected to the voltage -VA.

The combination consisting of the differential amplifier 3, thetransistors T3 and T4, the resistor Rg, the capacitor Cg and theresistor R5 is used to calculate the value of the average currentflowing through the switch K, when the latter is closed, and to comparethis value with the value of the collector current of the transistors T3or T4, this value being close to that of the current Ig and the currentgain of the current-mirror stages being assumed to be unity.

Thus:

    (Ia-Ig)Rg=(Ig-ie)R5

now Ia-Ig=ie, so that Ig=Ia-ie

It follows that:

    ie=Ia/((Rg/R5)+2)

The resistor Rg is preferably chosen with a very high value so as toform with the differential amplifier 3 an almost perfect integrator.

By giving the ratio Rg/R a high value, for example, about 100, itfollows that the current ie charging or discharging the capacitor Cduring the time T_(L) separating two sync line pulses has a much smallervalue (typically 100 times smaller in the example chosen) than the valueof the current Ia. Advantageously, the amplification circuit accordingto the invention has an input current ie whose average value is verysmall compared to the input current Ia of the amplifier A2. Theparasitic voltage ΔV=ie×T_(L) /C which appears on the input of theamplifier A2 is consequently very small. By way of example, for a 40 nFcapacitor C, a 20 μA current Ia and an Rg/R5 ratio of about 100, thevalue of the parasitic voltage ΔV is about 0.3 mV.

Preferably, the amplifier A2 is a current-feedback amplifier produced,for example, in bipolar technology.

Advantageously, the closed-loop bandwidth, for example having a valueequal to 150 MHz, is then independent of the value of the gain of theamplifier, the current consumption is low, about from 5 to 15 mA and thesupply voltages +VA and -VA can have low values, respectively +5 V and-5 V.

The capacitor CD which connects the negative input e(-) of thedifferential amplifier 3 to earth has a value chosen so as to moderatethe increase in the apparent impedance of the input e(-) when thefrequency increases. Thus, in order for the input e(-) of the amplifierA3 to be equivalent to a virtual earth in dynamic mode, the capacitor CDmust have, by way of example, a value of greater than or equal to 10times the value of the capacitor C.

According to the preferred embodiment of the invention described above,the output voltage VS2 is equal to zero volts. However, the inventionrelates to other embodiments in which the voltage VS2 is different fromzero. The input e(+) of the differential amplifier 3 is then no longerconnected to the earth of the circuit but to a voltage source having asvalue the value to which the voltage VS2 must be stabilized.

FIG. 4 represents a detailed description, according to the preferredembodiment of the invention, of an amplification circuit applied to avoltage stabilization circuit.

The voltage VS2 from a current-feedback amplifier, such as the amplifierA2 mentioned previously, is stabilized to zero volts using acompensation circuit which acts simultaneously on the input current Iaand on the offset voltage of the amplifier.

The elements making up the circuit in FIG. 4 are partly elementsidentical to those in FIG. 3 and partly new elements which do not appearin FIG. 3.

The elements identical to those in FIG. 3 are the amplifiers 3 and A2,the capacitors Cg and CD, the resistors Rg, R1, R2, R3, R4 and R5 andthe transistors T1, T2, T3 and T4. These elements are connected togetherin the same way as in FIG. 3.

The new elements are the capacitor C_(L) and the resistors Ri and RL.

The resistor Ri connects the output of the amplifier A2 to the inpute(-) of the differential amplifier 3.

As is known to those skilled in the art, the capacitor C_(L) and theresistor RL form a circuit usually placed on the input side of theamplifier A2 so as to eliminate, for example, an unnecessary and/orabnormally high DC component associated with the useful signal. For thispurpose, the video signal VE is applied to a first plate of thecapacitor C_(L), the second plate of which is connected to the input ofthe amplifier A2, and the resistor RL has a first terminal connected tothe second plate of the capacitor C_(L) and a second terminal connectedto the earth of the circuit.

The apparent input current of the circuit shown in FIG. 4, also notedhere by ie, flows through the capacitor C_(L). In the same way as in theapplication described in FIG. 3, the current ie flows through theresistor Rg and the calculation of the current ie leads to:

    ie=Ia/((Rg/R5)+2).

Thus, for values of Ia, Rg and R5 such as those mentioned above, thecurrent ie is a very small current.

According to the invention, the value of the resistor RL is very muchgreater than the value that this same resistor has according to theprior art. By way of example, the value of RL may be equal to 100 kΩ.The values of the resistors Rg, Ri and R5 may then be respectively equalto 1 MΩ, 100 kΩ and 10 kΩ.

The stabilization of the output voltage VS2, due to the action on theoffset voltage of the amplifier A2, takes place by means of the resistorRi.

According to the invention, the offset voltage of the circuit shown inFIG. 4 is equal to:

    V.sub.off =R5×Ia/[Rg/Ri]

Advantageously, the value of V_(off) is thus very small.

The ratio Rap of the offset voltage according to the invention to theoffset voltage according to the circuits known in the prior art is equalto:

    Rap=Ri×R5/Rg×RL

For the numerical values mentioned above, it follows that Rap=1%.

Because of the increase in the resistance of the resistor RL, for valuesof C_(L) identical to those in the prior art, the low cut-off frequencyof the connection established by the elements RL and C_(L) isadvantageously lower according to the invention than according to theprior art.

In order to ensure that the looped system described in FIG. 4 isstabilized, it is important that the following inequality be satisfied:##EQU1##

According to the preferred embodiment of the invention described in FIG.4, the voltage VS2 is stabilized to zero volts. The invention alsorelates to embodiments in which the voltage VS2 is stabilized to avoltage other than zero. The input e(+) of the differential amplifier 3is then no longer connected to the earth of the circuit but to a voltagesource having as value the value to which the voltage VS2 must bestabilized.

FIG. 5 represents an improvement of a part common to the circuits shownin FIGS. 3 and 4.

According to this improvement, the first terminal of the resistor Rg andthe first terminal of the capacitor Cg are not connected to the emittersof the transistors T3 and T4.

The first terminal of the resistor Rg and consequently the firstterminal of the capacitor Cg are connected, on the one hand, to thefirst terminal of a resistor R6, the second terminal of which isconnected to the earth of the circuit, and, on the other hand, to thecathode and to the anode of the two diodes D3 and D4, the respectiveanode and cathode of which are connected together and to the output ofthe differential amplifier 3 which is itself connected to the bases ofthe transistors T3 and T4.

The resistor R6 has a value more or less equal to that of the resistorR5 (not shown in FIG. 5). The diodes D3 and D4 make it possible tocompensate for the conduction threshold of the two transistors T3 and T4(not shown in FIG. 5).

All the elements R1, R2, R3, R4, R5, D1, D2, T1, T2, T3, T4 and A2 maybe produced so as to be combined on a single monolithically integratedcomponent.

According to the embodiments of the invention which are shown in FIGS. 3and 4, the single monolithically integrated component must have 3contacts, namely the input of the amplifier A2, the common bases of thetransistors T3 and T4, and the common emitters of the transistors T3 andT4.

Advantageously, according to the improvement shown in FIG. 5, the singlemonolithically integrated component must now have only 2 contacts,namely the input of the amplifier A2 and the common bases of thetransistors T3 and T4.

We claim:
 1. Amplification circuit, comprising:an amplifier (A2) havingan input and an output; a capacitor (C, C_(L)) having a first plate anda second plate, the second plate of the capacitor being connected to theinput of the amplifier and the first plate being connected to an inputvoltage (VE), an average current ie flowing through the capacitor andthe amplifier having as input current a current Ia; and means for makingthe average current ie flowing through the capacitor more or less equalto zero, whatever the value of the current Ia, the making meansconsisting of a current generator (1) generating a current Ig, and adevice (2) for measuring the average current ie, so that ie=Ia-Ig, thecurrent generator having first and second current generators, the firstcurrent generator including a pnp-type transistor T1, a resistor R1having a first terminal and a second terminal, a resistor R2 having afirst terminal and a second terminal, and a diode D1, the emitter of thetransistor T1 being connected to the first terminal of the resistor R1,the second terminal of the resistor R1 being connected to a positivesupply voltage +VA for the amplifier (A2), the base of the transistor T1being connected to the cathode of the diode D1, the anode of the diodeD1 being connected to the first terminal of the resistor R2, the secondterminal of of the resistor R2 being connected to the positive supplyvoltage +VA, the collector of the transistor T1 being connected to theinput of the amplifier (A2), and the second current generator includingan npn-type transistor T2, a resistor R3 having a first terminal and asecond terminal, a resistor R4 having a first terminal and a secondterminal, and a diode D2, the emitter of the transistor T2 beingconnected to the first terminal of the resistor R3, the second terminalof the resistor R3 being connected to a negative voltage -VA for theamplifier (A2), the base of the transistor T2 being connected to theanode of the diode D2, the cathode of the diode D2 being connected tothe first terminal of the resistor R4, the second terminal of theresistor R4 being connected to the negative voltage -VA. 2.Amplification circuit according to claim 1, characterized in that thecurrent-measuring device (2) includes a differential amplifier (3) witha positive input e(+) and a negative input e(-), a resistor Rg having afirst terminal and a second terminal, a capacitor Cg having a firstterminal and a second terminal, an npn-type transistor T3, a pnp-typetransistor T4 and a resistor R5 having a first terminal and a secondterminal, the positive input e(+) being connected to the earth of thecircuit, the negative input e(-) being connected to the first terminalof the capacitor Cg and to the first terminal of the resistor Rg, thesecond terminal of the capacitor Cg and the second terminal of theresistor Rg being both connected to the first terminal of the resistorR5, the second terminal of which is connected to the earth of thecircuit, the output of the differential amplifier being connected to thebases of the transistors T3 and T4, the emitters of which are connectedto the first terminal of the resistor R5, the collector of thetransistor T3 being connected to the base of the transistor T1 and thecollector of the transistor T4 being connected to the base of thetransistor T2.
 3. Amplification circuit according to claim 1,characterized in that the current-measuring device (2) includes adifferential amplifier (3) with a positive input e(+) and a negativeinput e(-), a resistor Rg having a first terminal and a second terminal,a capacitor Cg having a first terminal and a second terminal, acombination of two diodes (D3, D4) mounted back to back and constitutinga dipole having a first terminal and a second terminal, an npn-typetransistor T3, a pnp-type transistor T4 and a resistor RS having a firstterminal and a second terminal, the positive input e(+) being connectedto the earth of the circuit, the negative input e(-) being connected tothe first terminal of the capacitor Cg and to the first terminal of theresistor Rg, the second terminal of the capacitor Cg and the secondterminal of the resistor Rg being both connected to the first terminalof the said dipole, the second terminal of which is connected to thebases of the transistors T3 and T4, the output of the differentialamplifier being connected to the bases of the transistors T3 and T4, theemitters of which are connected to the first terminal of the resistorR5, the collector of the transistor T3 being connected to the base ofthe transistor T1 and the collector of the transistor T4 being connectedto the base of the transistor T2.
 4. Amplification circuit according toclaim 1, characterized in that it includes a switch (K) having a firstterminal and a second terminal, the first terminal of the switch (K)being connected to the input of the amplifier (A2) and the secondterminal of the switch (K) being connected to the negative input e(-) ofthe differential amplifier (3).
 5. Amplification circuit according toclaim 1, characterized in that it includes a resistor Ri placed betweenthe output of the amplifier (A2) and the negative input e(-) of thedifferential amplifier (3).
 6. Amplification circuit according to claim1, characterized in that it includes a capacitor (CD) placed between thenegative input e(-) of the differential amplifier (3) and the earth ofthe circuit.
 7. Circuit for restoring the DC component of a videosignal, characterized in that it includes an amplification circuitaccording to claim
 4. 8. Voltage stabilization circuit, characterized inthat it includes an amplification circuit according to claim
 5. 9.Amplification circuit according to claim 2, characterized in that itincludes a switch (K) having a first terminal and a second terminal, thefirst terminal of the switch (K) being connected to the input of theamplifier (A2) and the second terminal of the switch (K) being connectedto the negative input e(-) of the differential amplifier (3). 10.Amplification circuit according to claim 3, characterized in that itincludes a switch (K) having a first terminal and a second terminal, thefirst terminal of the switch (K) being connected to the input of theamplifier (A2) and the second terminal of the switch (K) being connectedto the negative input e(-) of the differential amplifier (3). 11.Amplification circuit according to claim 2, characterized in that itincludes a resistor Ri placed between the output of the amplifier (A2)and the negative input e(-) of the differential amplifier (3). 12.Amplification circuit according to claim 3, characterized in that itincludes a resistor Ri placed between the output of the amplifier (A2)and the negative input e(-) of the differential amplifier (3). 13.Amplification circuit according to claim 2, characterized in that itincludes a capacitor (CD) placed between the negative input e(-) of thedifferential amplifier (3) and the earth of the circuit. 14.Amplification circuit according to claim 3, characterized in that itincludes a capacitor (CD) placed between the negative input e(-) of thedifferential amplifier (3) and the earth of the circuit. 15.Amplification circuit according to claim 4, characterized in that itincludes a capacitor (CD) placed between the negative input e(-) of thedifferential amplifier (3) and the earth of the circuit. 16.Amplification circuit according to claim 5, characterized in that itincludes a capacitor (CD) placed between the negative input e(-) of thedifferential amplifier (3) and the earth of the circuit.
 17. Circuit forrestoring the DC component of a video signal, characterized in that itincludes an amplification circuit according to claim
 6. 18. Voltagestabilization circuit, characterized in that it includes anamplification circuit according to claim
 6. 19. Amplification circuit,comprising:an amplifier having an input and an output; a capacitor,having a first plate and a second plate, the second plate of thecapacitor being connected to the input of the amplifier and the firstplate of the capacitor being connected to an input voltage, thecapacitor having an average current flowing therethrough and theamplifier having an input current; means for making the average currentsubstantially equal to zero, the making means including a currentgenerator for generating a first current, and a measuring device formeasuring the average current so that the average current equals theinput current minus the first current; and a switch which enables duringa first period of time, the measuring of the average current, and duringa second period of time, the generating of the first current as afunction of a previously measured average current.
 20. Amplificationcircuit according to claim 19, wherein the current generator comprises:afirst current generator including a first transistor having a base, anemitter and a collector, the collector of the first transistor beingconnected to the input of the amplifier, a first resistor having a firstterminal and a second terminal, the emitter of the first transistorbeing connected to the first terminal of the first resistor, the secondterminal of the first resistor being connected to a positive supplyvoltage for the amplifier, a second resistor having a first terminal anda second terminal, the first terminal of the second resistor beingconnected to the positive supply voltage, and a first diode having ananode and a cathode, the base of the first transistor being connected tothe cathode of the first diode, the anode of the first diode beingconnected to the second terminal of the second resistor; and a secondcurrent generator includinga second transistor having a base, an emitterand a collector, a third resistor having a first terminal and a secondterminal, the first terminal of the second resistor being connected tothe emitter of the third transistor, the second terminal of the thirdresistor being connected to a negative supply voltage for the amplifier,a fourth resistor having a first terminal and a second terminal, thefirst terminal of the fourth resistor being connected to the negativesupply voltage, and a second diode having an anode and a cathode, thebase of the second transistor being connected to the anode of the seconddiode, the cathode of the second diode being connected to the secondterminal of the fourth resistor.
 21. Amplification circuit accordingclaim 20, wherein the measuring device includes:a differential amplifierwith a positive input and a negative input, the positive input beingconnected to the earth amplification of the circuit; a fifth resistorhaving a first terminal and a second terminal; a second capacitor havinga first terminal and a second terminal, the first terminal of the secondcapacitor being connected to the negative input and to the firstterminal of the fifth resistor; a third transistor and a fourthtransistor, each of the third and fourth transistors having a base, anemitter and a collector, the output of the differential amplifier beingconnected to the bases of the third and fourth transistors, the emittersof the third and fourth transistors being connected to the firstterminal of a sixth resistor, the collector of the third transistorbeing connected to the base of the first transistor and the collector ofthe fourth transistor being connected to the base of the secondtransistor; and a sixth resistor having a first terminal and a secondterminal, the second terminal of the second capacitor and the secondterminal of the fifth resistor being both connected to the firstterminal of the sixth resistor, the second terminal of the sixthresistor being connected to the earth of the amplification circuit. 22.Amplification circuit according to claim 20, wherein the measuringdevice includes:a differential amplifier having a positive input and anegative input, the positive input being connected to the earth of theamplification circuit; a fifth resistor having a first terminal and asecond terminal; a second capacitor having a first terminal and a secondterminal, the negative input being connected to the first terminal ofthe second capacitor and to the first terminal of the fifth resistor; acombination of two diodes mounted back to back and constituting a dipolehaving a first terminal and a second terminal, the second terminal ofthe second capacitor and the second terminal of the fifth resistor beingconnected to the first terminal of the dipole; a third transistor and afourth transistor, each of the third and fourth transistors having abase, an emitter and a collector, the second terminal of the dipolebeing connected to the bases of the third and fourth transistors, theoutput of the differential amplifier being connected to the bases of thethird and fourth transistors, the collector of the third transistorbeing connected to the base of the first transistor and the collector ofthe fourth transistor being connected to the base of the secondtransistor; and a sixth resistor having a first terminal and a secondterminal, the emitters of the third and fourth transistors beingconnected to the first terminal of the sixth resistor.
 23. Amplificationcircuit according to claim 22, wherein the switch has a first terminaland a second terminal, the first terminal of the switch being connectedto the input of the amplifier and the second terminal of the switchbeing connected to the negative input of the differential amplifier. 24.Amplification circuit according to claim 22, further comprising a thirdcapacitor placed between the negative input of the differentialamplifier and the earth of the circuit.
 25. Circuit for restoring the DCcomponent of a video signal, wherein the circuit includes anamplification circuit according to claim
 23. 26. Amplification circuit,comprising:an amplifier having an input and an output; a capacitorhaving a first plate and a second plate, the second plate of thecapacitor being connected to the input of the amplifier and the firstplate being connected to an input voltage, the capacitor having anaverage current flowing therethrough, the amplifier having an inputcurrent; and current generator means for generating a first current formaking the average current flowing through the capacitor substantiallyequal to zero, the current generator means including a first currentgenerator providing a positive current, a second current generatorproviding a negative current, and a device for measuring the averagecurrent and for controlling the current generator means so that theaverage current equals the input current minus the first current. 27.Amplification circuit according to claim 26, wherein the first currentgenerator includes:a first transistor having a base, an emitter and acollector, the collector of the first transistor being connected to theinput of the amplifier; a first resistor having a first terminal and asecond terminal, the emitter of the first transistor being connected tothe first terminal of the first resistor, the second terminal of thefirst resistor being connected to a positive supply voltage for theamplifier; a second resistor having a first terminal and a secondterminal, the first terminal of the second resistor being connected tothe positive supply voltage; and a first diode having an anode and acathode, the base of the first transistor being connected to the cathodeof the first diode, the anode of the first diode being connected to thesecond terminal of the second resistor; and the second current generatorincludes:a second transistor having a base, an emitter and a collector;a third resistor having a first terminal and a second terminal, thefirst terminal of the third resistor being connected to the emitter ofthe second transistor, the second terminal of the third resistor beingconnected to a negative supply voltage for the amplifier; a fourthresistor having a first terminal and a second terminal, the firstterminal of the fourth resistor being connected to the negative supplyvoltage; and a second diode having an anode and a cathode, the base ofthe second transistor being connected to the anode of the second diodewhose cathode is connected to the second terminal of the fourthresistor.
 28. Amplification circuit according to claim 27, wherein thedevice includes:a differential amplifier with a positive input and anegative input, the positive input being connected to the earth of theamplification circuit; a fifth resistor having a first terminal and asecond terminal; a second capacitor having a first terminal and a secondterminal, the first terminal of the second capacitor being connected tothe negative input and to the first terminal of the fifth resistor; athird transistor and a fourth transistor, each of the third and fourthtransistors having a base, an emitter and a collector, the output of thedifferential amplifier being connected to the bases of the third andfourth transistors, the emitters of the third and fourth transistorsbeing connected to the first terminal of a sixth resistor, the collectorof the third transistor being connected to the base of the firsttransistor and the collector of the fourth transistor being connected tothe base of the second transistor; and a sixth resistor having a firstterminal and a second terminal, the second terminal of the secondcapacitor and the second terminal of the fifth resistor being bothconnected to the first terminal of the sixth resistor, the secondterminal of the sixth resistor being connected to the earth of thecircuit.
 29. Amplification circuit according to claim 27, wherein thedevice includes:a differential amplifier having a positive input and anegative input, the positive input being connected to the earth of theamplification circuit; a fifth resistor having a first terminal and asecond terminal; a second capacitor having a first terminal and a secondterminal, the negative input being connected to the first terminal ofthe second capacitor and to the first terminal of the fifth resistor; acombination of two diodes mounted back to back and constituting a dipolehaving a first terminal and a second terminal, the second terminal ofthe second capacitor and the second terminal of the fifth resistor beingboth connected to the first terminal of the dipole; a third transistorand a fourth transistor, each of the third and fourth transistors havinga base, an emitter and a collector, the second terminal of the dipolebeing connected to the bases of the third and fourth transistors, theoutput of the differential amplifier being connected to the bases of thethird and fourth transistors, the collector of the third transistorbeing connected to the base of the first transistor and the collector ofthe fourth transistor being connected to the base of the secondtransistor; and a sixth resistor having a first terminal and a secondterminal, the emitters of the third and fourth transistors beingconnected to the first terminal of the sixth resistor.
 30. Amplificationcircuit according to claim 28, wherein the switch has a first terminaland a second terminal, the first terminal of the switch being connectedto the input of the amplifier and the second terminal of the switchbeing connected to the negative input of the differential amplifier. 31.Amplification circuit according to claim 28, further including a seventhresistor placed between the output of the amplifier and the negativeinput of the differential amplifier.
 32. Amplification circuit accordingto claim 28, wherein the amplification circuit includes a thirdcapacitor placed between the negative input of the differentialamplifier and the earth of the amplification circuit.
 33. Circuit forrestoring the DC component of a video signal, wherein the circuitincludes an amplification circuit according to claim
 30. 34. Voltagestabilization circuit including an amplification circuit according toclaim 31.